I don' t think it should scare the geese, but sometimes you need to use the most realistic decoys to get better results. Maybe the big feet and your shells looked more realistic in appearance than the whole spread. I wouldn' t chalk it up to magnums and standards mixed, ducks and geese can' t tell size well at all from the air because they see everything with one eye as they turn their heads, which doesn' t afford them good depth perception.